1. Field of the Invention and Brief Description of Prior Art
The piezo effect has been known for over 100 years, and consists in the production of an electrical charge by a piezoelectrically active material, in most cases a crystal, under the influence of a force. This effect has been used for about 50 years to manufacture mechanical-electrical converters. Acceleration transducers and pressure transducers are very well known. Force transducers were not used more frequently until electronics had advanced sufficiently for the low piezoelectric charges to be stored for long periods of time without losses. However, acceleration and pressure transducers are effectively and essentially force transducers in which the pressure or acceleration signal is converted to a force signal. Since in the observation of acceleration processes and pressure processes the variations, i.e. the dynamics, of the measured value are of primary interest, the storage of the piezoelectric charges did not play such an important part in these measurements as in force measurements, where statics are extremely important. The advantages of the piezoelectric measuring method are the high sensitivity, good linearity, long-term constancy and low temperature dependence. There have therefore been many attempts to use this method for static, or at least for quasistatic, force measurements too. Within the framework of the state of the art, reference is made to the book "Piezoelektrische Messtechnik" (Piezoelectric metrology) by J. Tichy and G. Gautschi. A force transducer is explained in principle on page 150, FIG. 8.4., and two practical designs of force transducers are shown in a sectional view of page 155, FIG. 8.6. These are force transducers which make use of the longitudinal direct piezo effect. The piezo element is a quartz plate, with the force to be measured acting perpendicularly to the plane of the plate. Such force transducers operate satisfactorily as long as the force to be measured exceeds a value of one or two deca N. For measuring forces of 1N or less, the piezoelectric charge generated is so low that losses in the electronics and disturbances from outside influences may impair the measurements. Higher piezoelectric charges are obtained if the transversal piezoelectric effect is used. Under this effect the force does not act perpendicularly but parallel to the electrode planes. By using oblong plates or rods with a small cross-section, a piezo effect many times greater than the longitudinal effect can be achieved. In the book mentioned, at page 156, FIG. 8.7, a pressure transducer is shown which also serves as a force transducer based on the transverse piezoelectric effect. Higher sensitivities can also be obtained in force transducers due to the higher piezoelectric charges emitted by transducers with transverse effect.
In piezoelectric transducers, the introduction or application of the force creates a problem which must be carefully solved. Most piezoelectric materials, such as quartz, tourmaline or piezoceramics, are brittle. The force must therefore be applied uniformly over surfaces which are as large, and flat as possible, with low specific surface pressure, so that no point loads are applied to produce cracks. Stringent requirements must therefore be imposed on the flatness of the force introduction and electrode surfaces. For a particular force, the piezoelectrically emitted charge is not exactly the same in cases where it is introduced directly or over a large area, at the edge or in the centre of a piezo element.
The calibration factors of piezoelectric transducers of the same design therefore vary by a few percent. It has been shown to be appropriate to subject the piezoelectric elements in transducers to an initial compressive stress so that the effective area of contact of the force receiving elements and the electrodes with the piezo elements is as large as possible, and does not vary substantially when the measuring force varies. This also reduces the variation in calibration factors from one transducer to another. Many piezoelectric materials, e.g. quartz and tourmaline, are hydrophylic to a certain extent, i.e. the surfaces which should provide insulation are covered with a thin layer of water which impairs the insulation. It is therefore normal practice to pack such materials into a housing to keep atmospheric moisture from penetrating the insulating surfaces. However, this housing acts as a mechanical shunt for the measuring force, which diverts some of this force in parallel with the piezo element, thereby reducing the sensitivity of the transducer. An initial stress also has a positive effect on this shunt because a support located between force introduction surfaces and piezo elements with a small effective area of contact exhibits considerable interlaminar elasticity, which increases the efficiency of the shunt through the housing, and also gives rise to considerable nonlinearity. By applying an initial stress, the so-called "shrinkage" of the calibration curve can be avoided. This "shrinkage" means that in the case of low measured values, the beginning of the curve is much steeper than for higher measured loads. Another important advantage of initial stress is the increase in strength, which is already known from the behaviour of prestressed concrete. Like concrete, most piezoelectric materials share the property that their compressive strength is much greater (10 to 50 times) than their tensile strength. By superimposing an initial compressive stress, no tensile stress is generated in the case of an external additional load, e.g. during bending. Instead the initial compressive stress is reduced at individual points. The limits of the loads which may be applied externally are substantially widened as a result of this. A further important advantage of the initial stress lies in the fact that where the force is introduced positively, i.e. if plates are merely placed on top of each other, or rods are only installed between stops, tensile and shearing forces cannot be transmitted, or even measured at all, because of raising and sliding actions. The immobility or rigidity is required for introducing and transmitting eccentric, possibly inclined compresssive forces or bending moments, and is obtained initially indirectly as a result of the initial stress itself, and the frictional forces of the different parts against one another. Most piezoelectric transducers used today are therefore provided with a prestressing element for generating such an initial stress. In the case of the force transducers, FIG. 8.6, page 155, in the book quoted above, by Tichy and Gautschi, this element is the housing (3). The pressure transducer, FIG. 8.7, page 156, is also provided with a clamping sleeve (2). The accelerameter, FIG. 8.8, page 156, is also provided with a clamping sleeve (2). In the force transducer in FIG. 9.1, page 162, the housing (1) is again the prestressing element. Most acceleration transducers are also provided with prestressing elements, e.g. FIG. 11.9, page 205a, where the housing acts as the prestressing element. Variants b, c and d have a central bolt as the prestressing element.
There is no doubt that prestressing gives many advantages, but there are also disadvantages. One lies in the fact that in order to generate the initial stress, an element is required which should record only the force to be measured, but necessarily also exerts a mechanical force shunt to the piezo element. This shunt has a detrimental effect on the temperature stability of the transducer.
In the case of temperature variations, the shunt does not expand to exactly the same degree as or at the same time as the piezo element, which gives rise to a variation in the initial stress, which is in turn expressed in the form of a zero point drift. Even if the shunt is successfully manufactured from material with exactly the same coefficient of expansion as the piezo element, there may nevertheless be differences in expansion in the case of inhomogeneous temperature distribution. This zero point shift due to temperature variation is called the pseudyo-pyro effect. By contrast, the real pyro effect is a well known phenomenon which is exhibited by certain piezoelectric materials, e.g. tourmaline or piezo ceramics, where the material itself generates a charge when the temperature varies. In most cases this effect is so great that absolutely no sensitive measurements are possible with such materials because the slightest temperature variations cause an excessive shift in the zero point. However, there are classes of piezo electric crystals, e.g. quartz, which have no pyro effect. A loose piezo element of such a material does not produce a charge when subjected to homogeneous heating. The situation is different, however, if the heating is applied locally, irregularly or as a temperature shock, and internal stresses are exerted. These internal stresses should not actually generate charges, if the piezo effect were to be the same at all points, because charges of both signs, which should compensate each other externally, are generated due to the enclosed nature of the non-positive force. But since the internal central sections of a piezo element are less piezoelectric than sections close to the surface of the material, because of the suppression of the Poisson effect, the compensation is not complete, with the result that residual charges may also be generated due to internal stresses. Moreover, it is very important how the electrodes are distributed, and whether they are able to record the charges generated or not. For example, it might be the case that tensile stresses are produced in the vicinity of an electrode, and that no electrode is present close to points where the corresponding compressive stresses are exerted, with the result that only the tensile stresses are recorded.